Interpretive Summary: Onion plants (Allium cepa) synthesize antioxidative flavonols, a class of flavonoids, to protect the cells against damaging effects of ultraviolet radiation of the sun and against internally produced hydrogen peroxide. Fresh and dehydrated onions are widely used in the human diet, not only as a spicy garnish but as a source of nutrients and non-nutritive health-promoting compounds. Flavonoids possess numerous beneficial bioactive properties that may also benefit human health, including antimicrobial activities against pathogenic bacteria. As part of an effort designed to relate the structures of these compounds to antimicrobial effects against foodborne pathogens, we determined, in collaborative studies carried out in Korea and Japan, the amounts of seven flavonoids in onion bulbs, in seven scales of an onion bulb, in scales subjected to ‘cooking’ methods used in the home, in scales exposed to fluorescent light, and in dehydrated widely consumed commercial onion products. Except for frying, home processing of onions did not significantly affect their flavonoid content. Exposure of fresh onions to fluorescent light increased flavonoid levels. An unexpected finding was the very low flavonoid content in commercial dehydrated onion products we are currently evaluating for antimicrobial properties. These findings may help consumers select processing methods that minimize loss of beneficial onion flavonoids.

Technical Abstract:
Onion plants synthesize flavonoids as protection against damage by UV-radiation and by intracellular hydrogen peroxide. Because antioxidative flavonoids also exhibit health-promoting effects in humans, a need exists to measure their content in onions and in processed onion products. To contribute to our knowledge about the levels of onion flavonoids, we used HPLC and LC/MS to measure levels of seven quercetin and isorhamnetin glucosides in four Korean commercial onion bulb varieties and their distribution within the onion, in scales of field grown onions exposed to home processing or to fluorescent light, and in 16 commercial dehydrated onion products sold at retail in the United States. Small onions had higher flavonoid content per kg than large ones. There was a graduated decrease in the distribution of the flavonoids across an onion bulb from the 1st (outside) to the 7th (inner-most) scale. Losses of onion flavonoids subjected to ‘cooking’ (in %) ranged as follows: frying, 33; sautéing, 21; boiling, 14-20; steaming, 14; microwaving, 4; baking, 0. Exposure to fluorescent light for 24 and 48 h induced time-dependent increases in the flavonoid content. Commercial, dehydrated onionproducts contained low amounts or no onion flavonoids. The results extend our knowledge about the distribution of flavonoids in fresh and processed onions.